Low pressure discharge lamp

ABSTRACT

A low pressure discharge lamp of double tube structure comprises an outer bulb containing therein a sealed gas and having one end closed and an inner tube supported by an end plate to communicate with the interior of the outer bulb; receiving therein an electrode and restricting an electric discharging path. The end receiving therein the electrode of the inner tube is bonded with an adhesive on the surface on the side facing the outer bulb of the end plate and a part of an end other than said end of said inner tube is in contact to and supported by the end plate. The construction makes bonding operations of the inner tube to the end plate easy and provide a stable condition to the inner tube in an lamp assemblage and reduces lamp weight.

TECHNICAL FIELD

The present invention relates to a low pressure discharge lamp having aninner tube to restrict a discharge path in an outer bulb forming adischarge space.

BACKGROUND ART

A variety of proposals have been made for this kind of discharge lamp tosubstitute it for an incandescent lamp because the discharge lamp allowsits size-reduction into a compact form. The discharge lamp disclosed inJapanese Unexamined Patent Publication No. 44956/1982 is shown in FIG. 1as an example, in which there are provided two electrodes 3 passingthrough an end plate 2 closing the lower opening portion of an outerbulb 1, extending in the outer bulb 1 and each one end of inner tubes 4,formed into a U-shape, is fitted to each projection 11 through whicheach of the electrodes 3 passes, on account of which cross-talkdischarge possibly occurring between the electrodes 3 is prevented. Theother open end of the inner tubes 4 is in contact with the upper surfaceof the end plate 2 and a recess 5 for causing bridge of electricdischarge is formed in the vicinity of each of the open ends. On theother hand, the inner tubes 4 are held by a pressing plate 10 fitted tothe upper end of a supporting rod 9 secured to the end plate 2 so thatboth the open ends are in contact with the end plate 2. A referencenumeral 7 designates a layer of fluorescent material.

In the discharge lamp having the construction described above, since thesupporting rod 9 and the pressing plate 10, both needed to secure theinner tubes 1 are incorporated in the lamp as additional parts, thereare problems of increasing the number of parts; causing additionalmanagement for these parts, complicated structure of the lamp andincreasing manufacturing steps.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a low pressuredischarge lamp having double tube structure comprising an outer bulb andinner tubes, in which each open end part at the side of covering anelectrode of the inner tubes is bonded with an adhesive to an end plateat a position facing the outer bulb and the other each open end part,which does not cover the electrode, of the inner tubes is made incontact with the end plate thereby eliminating problems of complicatedmanagement of parts; complicated manufacturing steps and increase inweight of the discharge lamp, all resulting from use of additional partsto secure the inner tubes, in view of the circumstances as describedabove.

It is another object of the present invention to provide a low pressuredischarge lamp having double tube structure, in which an outer bulb,inner tubes and a fine pipe for air discharge are bonded with anadhesive on a surface of an end plate facing the outer bulb wherebymanufacturing work can be simplified, damage of parts to be bonded orbonding portions can be prevented and reliability on airtightness can beincreased.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 is an exploded perspective view of a conventional low pressuredischarge lamp in contrast to the present invention;

FIG. 2 is an exploded perspective view of an embodiment of the lowpressure discharge lamp according to the present invention;

FIG. 3 is a diagram showing the preparation of an inner tube shown inFIG. 2;

FIG. 4 is a diagram showing the preparation of the inner tube in FIG. 2;

FIG. 5 is a diagram showing the preparation of the inner tube in FIG. 2;

FIG. 6 is an exploded perspective view of another embodiment of the lowpressure discharge lamp according to the present invention;

FIG. 7 is a cross sectional view partly omitted of the other embodimentof the discharge lamp of the present invention;

FIG. 8 is a cross sectional view of an important part of still anotherembodiment of the present invention;

FIG. 9 is a cross sectional view showing another embodiment of thepresent invention; and

FIG. 10 is a cross sectional view of a separate embodiment of thedischarge lamp of the present invention.

BEST MODE OF CARRYING OUT THE INVENTION

An embodiment of the present invention will be described with referenceto FIG. 2.

A reference numeral 1 designates a cylindrical outer bulb made of glasshaving a domed top and an opened bottom and numeral 2 refers to an endplate made of metal or ceramics for closing the opened bottom of theouter bulb 1 to make it in airtight condition. There is enclosed amedium for discharge consisting of a predetermined amount of mercury anda rare gas such as argon in a sealed space formed by the end plate 2 andthe outer bulb 1. Numeral 3 designates electrodes both being seal-bondedto the end plate 2 to extend into the outer bulb 1 and a numeral 4indicates an inner tube made of glass which is to be arranged in theouter bulb 1 to restrict a discharge path. The inner tube 4 is formed inone piece by melt-bonding each of legs of two glass tubes bended to be aU-shape. Namely, free ends of the inner tube 4 are bonded with anadhesive of glass frit 8 to a surface of the end plate 2 in an air-tightfashion to respectively cover the electrodes 3, 3 so that no cross-talkelectric discharge takes place between the electrodes 3, 3. On the otherhand, the other ends of the inner tube 4 is bridged by formingtherebetween a connecting part 6 by melt-bonding the vicinity of theopen ends 5 of the two U-shaped glass tubes so that the end part thusconnected is brought to in contact with the surface of the end plate 2.A substantially semi-circular opening 5 formed by obliquely cutting theglass tube from its open end is provided in each side portion of theglass tubes facing each other.

Simple explanation will be made as to assembling work for the embodimentshown in FIG. 2.

First of all, an adhesive 8 is previously applied to on the end plate 2.

A single inner tube having two U-shaped parts formed in one piece byconnecting part 6 is set up on the surface of the end plate 2 with itsfour lower surfaces without any supporting aid. Then, the outer bulb 1is sealed by the end plate 2.

In the assembling work as above-mentioned, it is unnecessary to use ajig (not shown) for supporting the inner tube for the purpose of bondingthe inner tube 4 to the end plate 2 with the adhesive of glass frit 8,as required in the conventional processes to secure an inner tube 4consisting of a single U-shaped glass tube, whereby the assembling workcan be remarkably simplified. Further, since the four end surfaces ofthe inner tube 4 are all in contact with the end plate surface, it israre that the inner tube falls down even though an impact force isapplied to a incomplete lamp in the later stage of the assembling workor a completed lamp.

Incidentally, since the inner tube supporting rod as shown in FIG. 1 isnot required because the inner tube 4 is attached by bonding operation,the number of parts needed can be reduced, hence the number of themanufacturing steps can be reduced and accordingly, management for theparts is easy and manufacturing process is simplified.

The supporting rod 9 and the pressing plate 10 used in the conventionallamp as shown in FIG. 1 have been made of metal. In the discharge lampof this kind which is required to reduce its weight for the purpose ofsubstituting the discharge lamp for an incandescent lamp, if thesupporting rod 9 and the pressing plate 10 can be eliminated, the totalweight of the lamp can be reduced.

In the next place, a method of preparation of the inner tube 4 of theembodiment shown in FIG. 2 will be described. The inner tube 4 isprepared according to steps as shown in FIGS. 3 to 5. Two glass tubes A,A each having a U-shaped portion are placed in juxta position withrespect to two legs A₁, A₁ and A₂, A₂ facing each other and with theiropen ends directing upwards; the upper portions of the respective legsA₂, A₂ are simultaneously heated by a burner C from the open end to thejust lower portion to cause them to be molten; after a short timeperiod, a part of the top of the respective legs A₂, A₂ deforms due towind pressure of the flame from the burner C to form a semi-circularopening 5 as if cut obliquely from the open end and a mass of moltenglass 52 is formed at the peripheral portion 51 of the opening 5 asshown in FIG. 4. The molten glass mass formed at the lower part of theperipheral portion, but at the upper part when the inner tube 4 is setup on the end plate 2 is pinched and the molten glass mass of both theglass tubes A, A are brought into contact each other; then, the glasstubes A, A are slightly apart from each other to form a connecting part6; thus the preparation of the inner tube 4 is completed as shown inFIG. 5. It is advantageous that a fluorescent layer 7 to be coated onthe inner wall of the inner tube 4 is formed after the preparation ofthe inner tube 4 because the connecting part 6 can be easily formed. Inthe discharge lamp of the present invention, since the connecting part 6is formed at the same time of formation of the opening 5 as a recess tocause a bridge of electric discharge when the inner tube 4 is to beprepared, the number of steps for heating and annealing the glass tubeare reduced.

The shape of the opening 5 to be formed in the inner tube 4 is notlimited to a semi-circular one which is formed by obliquely cutting theleg from the open end, but can be a circular shape which may be formedat the intermediate of the leg of the inner tube 4. In this case, thecircular opening 5 can be obtained by blowing a flame oliquely from theoutside at a high temperature such as oxygen gas flame at a positionwhere an opening is to be formed in the leg.

The shape of the inner tube 4 is variable; for example, the inner tubeis formed in such a manner that a glass tube in a U-shape is againsubjected to bending operation into a saddle shape to form a pluralityof U-shaped portions. Then, the electrodes are covered by the respectiveends of the glass tube and the lower side surfaces of the plurality ofU-shaped portions are brought into contact with the end plate.

It is considered that in addition to the inner tube bonded to the endplate with an adhesive, the outer bulb and the fine tube for airdischarge are also bonded to the end plate with an adhesive. In thiscase, bonding operations are carried out by putting an adhesive of glassfrit in either a molded form or paste on predetermined portions; puttinga body to be bonded such as an outer bulb on the glass frit or directlyputting the body on, for example, the end plate, followed by coating apaste-like glass frit around the contacting area of the body, and thenapplying heat to the glass frit until the glass frit reaches apredetermined temperature for melt-bonding.

In the discharge lamp having many portions to be bonded with an adhesivesuch as glass frit, it is essential to reduce the number of bondingoperations to simplify manufacturing work. Further, when the inner tube,the outer bulb and the fine tube for air discharge are separatelybonded, a part having been bonded or bonding portion may be broken orair-tight property may be impaired due to thermal stress caused byapplication of heat in the bonding operations conducted later.

As measures to solve such drawback, there is another embodiment of thepresent invention which will be described with referece to FIG. 6 and 7.In FIG. 6, the same reference numerals as in FIG. 2 designate the sameor corresponding parts and therefore, the description is omitted.

FIG. 6 shows a discharge lamp in which there are provided two innertubes 4 each having an upper open end and the lower end bonded to theend plate 2 with an adhesive 8 such as glass frit and each enlosingtherein an electrode 3. The reference numeral 12 designates a fine tubefor air discharge which has one end communicated with the interior ofthe outer bulb 1 for the purpose of evacuation of air. The fine tube 12is fitted into an insertion hole 21 for fine tube formed in the endplate 2 as shown in FIG. 7.

The construction of the end plate 2 will be described in more detailwith reference to FIG. 7.

The insertion hole 21 for fine tube is provided between the electrode 3,3 in the end plate 2 and a recess 22 having relatively large diameter isformed in the surface at the side of the electrodes, of the end plate 2.Annular grooves 23 are formed around the electrodes 3 in the end plate 2so as to have a diameter corresponding to the inner tube 4. Separateannular grooves 24 are formed near the edge of the end plate 2 on thesame side as the annular grooves 23 as above-mentioned so as to have adiameter corresponding to the outer bulb 1. In these grooves 22, 23 and24, the adhesive 8 of glass frit is applied.

Assembling operations for the discharge lamp having the constructiondescribed above will be explained.

First of all, the adhesive 8 of paste-like glass frit is applied to eachof the grooves 22, 23 and 24 in the end plate 2. An end part of the finetube 7 for air discharge is inserted into the insertion hole 21 for finetube. The inner tube 4 is put on the glass frit 8 in the groove 23. Inthe same manner, the outer bulb 1 is put on the adhesive 8 in the groove24. Thus combined product is subject to preliminary drying and then itis passed through a heat oven to cause melt-bonding of the adhesive 8 ofglass frit followed by solidification; thus, the bonding operation iscompleted.

Since the grooves 22, 23 and 24 to which the outer bulb 1, the innertube 4 and the fine tube 12 for air discharge are bonded are allprovided on the single surface of the end plate 2, which faces the outerbulb 1, the assembling work such as application of the adhesive 8 ofglass frit, putting or inserting the outer bulb (1), the inner tube 4and the fine tube 12 and so on are all carried out on the same surfaceside without requirement of necessity of separate operations for theouter bulb (1) or the inner tube 4 etc. In this way, the assemblingoperation is simplified.

It is known that when a glass frit as an adhesive is heated, it becomesmolten state to exhibit fluidity. In the present invention, however,since the grooves 22, 23, 24 to which the glass frit is applied areformed in the same surface of the end plate 2, there is no risk offlowing the glass frit from any of grooves 22, 23, 24.

In the embodiment, it is unnecessary to separately carry out bondingoperations on account of which a part having been bonded is not damageddue to thermal stress caused by bonding operations to the other part andexcellent mechanical strength and air-tight property can be obtained.

Description will be made as to another embodiment of the presentinvention.

Although the fine tube 7 for air discharge of generally cylindricalshape is used in the embodiment as shown in FIGS. 6 and 7, it can be onewhich has an external flange 121 at one end of the fine tube whichextends in the outer bulb 1 as shown in FIGS. 8 and 9. This structure ofthe fine tube 12 eliminates an auxiliary means to support the fine tube12 for air discharge.

For a separate embodiment of the construction for the electrodes 3, 3and the end plate 2, it is possible, as shown in FIG. 10, to providesupporters 31, 31 made of glass or ceramics to support the electrodes 3,3 and bond the supporters 31, 31 with the glass frit on the end platesurface facing the outer bulb as with the other elements. Further, it isalways unnecessary to apply the glass frit in the grooves 22, 23, 24 asshown in FIG. 10.

Use of the glass frit as an adhesive is made not only in paste form asdescribed above but also in a predetermined molded form to a part or theentire portion of the end plate 2.

In the embodiments shown in FIGS. 6 to 10, it is, of course, possible touse the inner tube 4 in which two U-shaped glass tubes are combined intoone piece as shown in FIG. 2, or in which a single U-shaped tube is bentinto a saddle form.

We claim:
 1. A low pressure discharge lamp of double tube constructioncomprising:an outer bulb containing one open end with the remainder ofthe bulb being completely closed; an end plate placed in sealingrelationship with the open end of said outer bulb; a plurality ofelectrodes mounted on said end plate and sealed thereto; a gas containedwithin the volume defined by said outer bulb and said end plate; aninner tube including a plurality of light transmitting tubes, eachhaving an open end which receives therein one of said electrodes andwhich is adhesively bonded to and supported by the surface of said endplate; each of said light transmitting tubes restricting an electricdischarging path; each of said light transmitting tubes having at leastone opening in communication with said volume and having a part of anend other than said open end in contact with and supported by said endplate; said tubes being connected to each other by melt-bonding at apart of the edge portion of said opening.
 2. A low pressure dischargelamp of double tube construction comprising:an outer bulb containing oneopen end with the remainder of the bulb being completely closed; an endplate being adhesively bonded to the open end of said outer bulb; aplurality of electrodes mounted on said end plate and sealed thereto; agas contained within the volume defined by said outer bulb and said endplate; an inner tube having an open end which receives therein one ofsaid electrodes and which is adhesively bonded to and supported by thesurface of said end plate; said inner tube restricting an electricdischarging path; said inner tube having a part of an end other thansaid open end in contact with and supported by said end plate where saidpart of an end other than said open end is open in communication withsaid volume; and a fine tube for air discharge being adhesively bondedto said end plate.
 3. The low pressure discharge lamp according to claim2, wherein said inner tube, outer bulb and fine tube for air dischargeare bonded with an adhesive at respective grooves formed in said endplate.
 4. A low pressure discharge lamp of double tube constructioncomprising:an outer bulb containing one open end with the remainder ofthe bulb being completely closed; an end plate being adhesively bondedto the open end of said outer bulb; a plurality of electrodes mounted onsaid end plate and sealed thereto; a gas contained wihin the volumedefined by said outer bulb and said end plate; an inner tube having anopen end which receives therein one of said electrodes and which isadhesively bonded to and supported by the surface of said end plate;said inner tube restricting an electric discharging path; said innertube being in communication with said volume; and a fine tube for airdischarge having a flange formed at an end portion extending into saidouter bulb and being adhesively bonded to said end plate.
 5. A lowpressure discharge lamp of double tube construction comprising:an outerbulb containing one open end with the remainder of the bulb beingcompletely closed; an end plate being adhesively bonded to the open endof said outer bulb; a plurality of electrodes mounted on said end plateand being adhesively bonded thereto through an electrode supporter; agas contained within the volume defined by said outer bulb and said endplate; an inner tube having an open end which receives therein one ofsaid electrodes and which is adhesively bonded to and supported by thesurface of said end plate; said inner tube restricting an electricdischarging path; said inner tube being in communication with saidvolume; and a fine tube for air discharge being adhesively bonded tosaid end plate.